Generally, single transistors are not capable of delivering high power for output circuits and the like. Therefore, it is common to utilize a plurality of transistors in parallel for circuits requiring more power than single transistors can deliver. Also, connecting a plurality of transistors in parallel should reduce the amount that any one of the transistors conducts and, thus, should reduce the heat produced by individual transistors.
The problem is that the individual transistors and the accompanying circuits are not always evenly balanced. If one of the transistors in the plurality of parallel transistors starts to conduct more current for any reason, it will begin to heat more than the other transistors. It can be shown that the beta, or amount of conduction, of a transistor is dependent upon the heat of the transistor. Thus, as the transistor conducts more the temperature rises, which causes it to conduct more, and the conduction continues to spiral upward until one transistor is carrying all of the load and the others have virtually shut off.
This phenomenon is reduced in prior art devices by mounting the transistors on heat sinks, such as a chassis, to maintain the heat constant. There are several problems with this prior art solution. Mounting transistors on a chassis, for example, produces much vibration in the leads to the transistors, thereby causing lead breakage and a large number of failures. Further, it is difficult to mount the transistors on heat sinks, chassis and the like and requires many additional assembly steps which are costly and inefficient.